InGaN-based Laser Research Articles

Examination of thermal properties and degradation of InGaN - based diode lasers by thermoreflectance spectroscopy and focused ion beam etching

In this paper, thermal properties of InGaN-based diode lasers are investigated. The thermoreflectance technique was employed to study temperature distributions on the front facet of device. Measurements were performed, allowing investigation of the contribution of two main heat sources to the total temperature rise observed on the facet of device. It has been found that the contribution from reabsorption of laser emission at the facet, is much smaller than the one caused by Joule heating (electrical power). Additionally, devices have been investigated by means of SEM and FIB to determine the degradation sources. Inspection of the devices confirmed the lack of mirror damage or deposits. The main source of degradation was found to be located in the region of ridge and caused by extended defects. Our findings confirm the hypothesis that injected current is the major driving force of degradation.

Long-term degradation of InGaN-based laser diodes: Role of defects

The aim of this paper is to give an extensive presentation of the defect-related degradation of InGaN-based laser diodes (LDs) submitted to constant current stress, at room temperature. The analysis is based on combined electrical-optical characterization, the capacitance-temperature analysis, and deep-level transient spectroscopy (DLTS). The results show that stress induces a significant increase of the threshold current and the appearance of two defects, possibly related to point defects arranged along lines.

The effect of AlGaN bulk and AlGaN/GaN superlattice cladding layers on performance characteristics of deep violet InGaN DQW lasers

This paper reports performance characteristics of InGaN-based DQW laser diodes with different SLS cladding layer (CL) structures using the ISE TCAD software. LDs with SLSs as CLs were shown to have superior optical and electrical properties compared to LDs with bulk AlGaN CLs.The simulation results show the variation of output power, threshold current, slope efficiency, and differential quantum efficiency (DQE) from 13.51 to 10.88 mA, 20.92 to 41.27 mW, 1.83 to 1.87 W/A and 57.88 to 59.10%, for bulk to SLS CLs. Results show the increased operating current, electron and hole carrier densities and radiative recombination which enhanced output power and reduced threshold current for the structure with the SLS down CL. Using SLS in upper CL doesn't have significant effect on operating current, it increase the slope efficiency and DQE. Using SLS structure simultaneously in up and down CLs enhances the output power, DQE and slop efficiency, and decreases the threshold current.

Evolution of differential efficiency in blue InGaN laser diodes before and after a lasing threshold.

The optical power emitting from the cavity facet of blue InGaN-based laser diodes (LDs) is measured to investigate the efficiency droop. The efficiency droop behavior of blue InGaN-based LDs near the threshold is confirmed in our experiments. From measurements of optical power at different wavelengths, it is analyzed that the droop behavior of LDs can be ascribed to the efficiency reduction of longer wavelengths. The efficiency of longer wavelengths is subject to the carrier occupation process in quantum levels. In addition, it is found that the droop behavior may be largely affected by the relatively large threshold current of InGaN-based LDs and the screening effect of polarization, and it can be suppressed by stimulated emission.

Spectroscopy and visible laser operations of a μ-PD grown Pr3+ :LiYF4 single-crystal fiber

We report on the growth and visible laser operations of a single-crystal fiber of Pr3+ :LiYF4 (Pr:YLF), grown by the micro-pulling-down method (μ-PD). We performed spectroscopic analyses on the crystal and compared the results with a Czochralski-grown sample of the same material. These analyses proved the high optical quality of the fiber. Using a hemispherical cavity and pumping the crystal with an InGaN-based laser diode, we achieved laser emission in the orange, red and deep red regions. We measured the slope efficiencies of three transitions and studied the propagation of output beams. To the best of our knowledge, these are the first laser operations in a Pr:YLF single-crystal fiber grown by this method.

Degradation of InGaN laser diodes caused by temperature- and current-driven diffusion processes

In this paper, we analyze the degradation of InGaN-based green laser diodes submitted to stress tests at different bias currents and junction temperatures. The variation of the threshold current suggests the presence of a diffusion process. The fitting of the degradation kinetics according to Fick's second law allows for the extrapolation of the diffusion coefficient of the impurity/defect involved in the degradation process, and the related activation energy of 1.98eV. The diffusion process is accelerated not only by the temperature but also by the flow of current. Taking into account the strong difference between experimental and theoretical diffusion coefficients of Mg and H in GaN, the physical model of the diffusion may need to be revised in order to take into account the role of electric field and carrier flow. The extrapolated values of the diffusion coefficient and of the related activation energy are consistent with the hypothesis that the degradation originates from the diffusion of hydrogen in the H+ species.

Hole transport in c-plane InGaN-based green laser diodes

Hole transport in c-plane InGaN-based green laser diodes (LDs) has been investigated by both simulations and experiments. It is found that holes can overflow from the green double quantum wells (DQWs) at high current density, which reduces carrier injection efficiency of c-plane InGaN-based green LDs. A heavily silicon-doped layer right below the green DQWs can effectively suppress hole overflow from the green DQWs.

Room-temperature continuous-wave electrically injected InGaN-based laser directly grown on Si

The fabrication of a blue–violet semiconductor laser on a silicon substrate enhances the integration of optoelectronics with electronics.

Nanoscale Investigation of Degradation and Wavelength Fluctuations in InGaN-Based Green Laser Diodes

We present a study of the optical inhomogeneities and degradation of InGaN-based green laser diodes based on high-resolution cathodoluminescence (CL) investigation of the output facets and inner cross-section of the devices. The results indicate that 1) degradation originates from a diffusion process, which causes an increase in the threshold current. 2) Nanoscale-level CL points out the circular symmetry of the degraded area, which is wider than the ridge and includes not only the quantum wells but also the waveguiding and cladding layers. 3) The yellow luminescence decreases within the degraded region, whereas its intensity increases outside of the degraded region. 4) Wavelength fluctuations are found in both quantum wells and waveguides, which are critically analyzed and ascribed to inhomogeneity in indium concentration. Their possible effect in the filamentation of the laser emission is discussed.

The thickness design of unintentionally doped GaN interlayer matched with background doping level for InGaN-based laser diodes

In order to reduce the internal optical loss of InGaN laser diodes, an unintentionally doped GaN (u-GaN) interlayer is inserted between InGaN/GaN multiple quantum well active region and Al0.2Ga0.8N electron blocking layer. The thickness design of u-GaN interlayer matching up with background doping level for improving laser performance is studied. It is found that a suitably chosen u-GaN interlayer can well modulate the optical absorption loss and optical confinement factor. However, if the value of background doping concentration of u-GaN interlayer is too large, the output light power may decrease. The analysis of energy band diagram of a LD structure with 100 nm u-GaN interlayer shows that the width of n-side depletion region decreases when the background concentration increases, and may become even too small to cover whole MQW, resulting in a serious decrease of the output light power. It means that a suitable interlayer thickness design matching with the background doping level of u-GaN interlayer is significant for InGaN-based laser diodes.

Effectiveness of inserting an InGaN interlayer to improve the performances of InGaN-based blue-violet laser diodes

Electron leakage still needs to be solved for InGaN-based blue-violet laser diodes (LDs), despite the presence of the electron blocking layer (EBL). To reduce further electron leakage, a new structure of InGaN-based LDs with an InGaN interlayer between the EBL and p-type waveguide layer is designed. The optical and electrical characteristics of these LDs are simulated, and it is found that the adjusted energy band profile in the new structure can improve carrier injection and enhance the effective energy barrier against electron leakage when the In composition of the InGaN interlayer is properly chosen. As a result, the device performances of the LDs are improved.

Deep red diode-pumped Pr³⁺:KY₃F₁₀ continuous-wave laser.

We report the first observation of continuous-wave laser emission at 720 nm, in the deep red region, of a Pr3+:KY3F10 (Pr:KYF) single crystal, pumped with a blue InGaN-based laser diode at 446 nm. We employed a hemispherical cavity with three different output coupling mirrors transmitting 0.7%, 1.4%, and 2.7% of laser radiation. We obtained a maximum output power of 207 mW with a slope efficiency of 24.3%, comparable for the first time to what has been reported for other praseodymium-doped fluoride crystals. The round-trip cavity losses for our sample were estimated to be between 0.3% and 0.6%, a remarkably small value for this material.

InGaN based green laser diodes on semipolar GaN substrate

This paper reviews the development of the InGaN-based green laser diodes on semipolar GaN substrates, especially focusing on plane. The reduction of piezoelectric fields in InGaN quantum wells on the planes was investigated by a small blue shift in electroluminescence peaks, and high crystal quality was confirmed by clear interfaces in a transmission electron microscopy image, narrower FWHM in electroluminescence peaks, and smaller localization energy in time resolved PL results, as compared with the other planes. These physical characteristics of the leads to better laser properties: lower threshold current densities, higher output powers of over 100 mW in the spectral region beyond 530 nm, and higher wall plug efficiencies as high as 7.0–8.9% in the wavelength range of 525–532 nm, compared to those of the other planes. Estimated lifetimes were over 5000 h at 50 mW and 2000 h at 70 mW under cw operation with auto power control at a case temperature of 55 °C. The plane is the promising candidate for InGaN-based true green laser diodes.

Thermally-activated degradation of InGaN-based laser diodes: Effect on threshold current and forward voltage

This paper presents a study of the effects of high temperature stress on the electro-optical characteristics of violet InGaN-based laser diodes. The results indicate that: (i) when submitted to constant current stress (with relatively high junction temperatures), devices show a significant increase in threshold current (Ith), related to the increase in non-radiative recombination; micro-cathodoluminescence measurements indicate that the area affected by degradation is wider than the ridge; (ii) the results of purely-thermal stress test indicate that the exposure to high temperature may induce an increase in threshold current; (iii) during the first part of the stress, this mechanism is well correlated with the variation of the forward voltage, suggesting a degradation in the properties (conductivity, acceptor doping) of the p-type material; for longer stress times, a further Ith increase is detected, with a linear dependence on time.

Suppression of electron leakage by inserting a thin undoped InGaN layer prior to electron blocking layer in InGaN-based blue-violet laser diodes.

InGaN-based blue-violet laser diodes (LDs) suffer from electron leakage into the p-type regions, which could be only partially alleviated by employing the electron blocking layer (EBL). Here, a thin undoped InGaN interlayer prior to EBL is proposed to create an additional forbidden energy range above the natural conduction band edge, which further suppresses the electron leakage and thus improve the characteristics of LDs. Numerical device simulations reveal that when the proper composition and thickness of InGaN interlayer are chosen, the electron leakage could be efficiently eliminated without inducing any severe accumulation of electrons at the interlayer, resulting in a maximum output power of the device.

Variations in junction capacitance and doping activation associated with electrical stress of InGaN/GaN laser diodes

This paper presents a study of the effects of combined electrical and thermal stress tests on commercially-available InGaN-based blue laser diodes. By means of electrical and optical measurements, we have found that stress has two major consequences: (i) an initial decrease in threshold current, which is ascribed to the increase in the activation of p-type dopant, and (ii) a subsequent increase in threshold current, which – based on optical and electrical characterization – is ascribed to the generation/propagation of non-radiative defects within the active region of the devices.

Waveguide Optimization for Semipolar (In,Al,Ga)N Lasers

In this work the optical waveguiding in semipolar InGaN-based laser diodes is analyzed. Different designs of the separate confinement heterostructure with AlGaN or GaN cladding layers and GaN or InGaN waveguide layers are studied. The influence of waveguide material, thickness and composition on the optical confinement factor Γ, the accumulated strain energy E and the refractive index contrast is calculated. Measurements of the threshold and the far field intensity distributions of lasers with differing waveguide design confirm the predictions from model calculations. The optimum waveguide for a 410 nm single quantum well laser is found to consist of a symmetric In0.04Ga0.96N waveguide of 2×85 nm thickness with GaN cladding layers.

Improvement of characteristics of InGaN-based laser diodes with undoped InGaN upper waveguide layer

In the structure of conventional InGaN-based laser diodes, considerable optical absorption loss exists in the p-doped region, especially in the p-doped upper waveguide layer. A structure of InGaN-based laser diodes with an undoped InGaN upper waveguide layer instead of p-doped GaN was designed and optimized. Instead of being located between AlGaN EBL and AlGaN cladding layer, the undoped InGaN layer was positioned between the AlGaN electron blocking layer and the active region of multi-quantum well. The optical and electrical characteristics of this type of laser diodes were simulated by the commercial software lastip, and it was found that the optical absorption loss was obviously lowered. Further theoretical analysis showed that the undoped InGaN upper waveguide layer eliminated the influence of electron leakage current, leading to a reduction of threshold current density and an increase of output light power of the InGaN-based laser diodes.

Static and dynamic properties of multi-section InGaN-based laser diodes

We have studied multi-section InGaN multiple-quantum-well (MQW) laser diodes grown on c-plane freestanding GaN substrate consisting of an absorber section (AS) and an amplifier gain section. As a result of the interplay between external bias applied to the AS and the internal piezoelectric and spontaneous polarization fields inherent to c-plane InGaN MQWs, the devices exhibit non-linear non-monotonic variations of the threshold current due to the quantum-confined Stark effect that takes place in the AS MQWs. We report on how this effect tailors the lasing characteristics and lasing dynamics, leading from a steady-state cw lasing regime for an unbiased AS to self-pulsation and Q-switching regimes at high negative absorber bias.

Analysis of below-threshold efficiency characteristics of InGaN-based blue laser diodes

In this study, we investigate the below-threshold emission characteristics of InGaN-based blue laser diodes (LDs) emitting at 442 nm to study the efficiency droop effects in InGaN LDs. From the measurement of spontaneous emission in the LD, it is observed that the peak efficiency appears at a current density of ∼20 A/cm2 and the efficiency at the threshold current density of ∼2.3 kA/cm2 are reduced to ∼47% of the peak efficiency. The measured spontaneous emission characteristics are analyzed using the carrier rate equation model, and the peak internal quantum efficiency is found to be ∼75% using the fit of the measured efficiency curve. In addition, the Auger recombination coefficient of the measured InGaN blue LD is found to be 10−31–10−30 cm6/s, which is somewhat lower than that reported for InGaN-based blue light-emitting diodes. It is discussed that low dislocation density and uniform current injection in quantum wells may have resulted in the low Auger recombination coefficient of InGaN LDs.